County Bridges – Be Safe and Heed the Warnings
Agricultural and ag-business traffic is the primary use for many miles of county roadways and supporting that traffic is a major focus for the county’s road maintenance budget. However, budget limitations faced by county governments statewide prevent ideal conditions for roads and bridges. County bridges are inspected every 2 years and it is clear that there is an aging problem that requires constant attention.
A large majority of County bridges are built from treated timber, built in the 40’s, 50’s, and 60’s. The construction is relatively basic as far as bridges go. Timber pilings, similar to telephone poles, are pounded into the ground in rows perpendicular to the road. On top of the pilings, the bridge deck is built from a series of crisscrossing layers of planks.
Figure 1. Underside of Timber Bridge
Figure 2. Side of Timber Bridge
When the bridges were built, they were capable of supporting about any legal load, but over time, moisture and pests caused a range of timber decay, most prominently found in the pilings at the normal water level. Every year bridges are evaluated for remaining load bearing capacity. Weight limits are posted at the bridges, if necessary, according to those recommendations, typically 26 tons or less. In the interest of safety and structure preservation, all farmers, haulers, buses and other motorists are urged to obey the posted weight limits.
Figure 3. Typical Weight Limit Posting
Currently in Winnebago County, weight limit postings range from 26 tons to 8 tons and dramatically limit the use of those bridges. Many grain combines weigh between 15 and 16 tons depending on make and model with some reaching nearly 20 tons. Large tractors pulling implements are very similar. Even a loaded school bus can reach 16 tons or more. Most importantly, loaded semis, fertilizer applicators, tank wagons, grain carts, and other material-loaded vehicles can exceed weight postings by double or more. The safe strategy is to discontinue use of posted bridges until they are replaced, but obtaining definite equipment weights and working in cooperation with the County Engineer’s office is the most effective way to work through the inconvenience.
In most cases, the bridge decks are in good condition, potentially creating a false opinion of stability. Conversely, in many cases, one or more of the pilings underneath are entirely rotten or broken like an old fence post. In other cases, all pilings have significant decay. The posted weight limits are not only legally applicable to everyone including farmers, but also the threshold for which a heavier load could collapse the bridge. Weight limits are difficult to enforce, but violating the safe limit is not worth risking a life.
Figure 4. Broken Piling under a 10 TN Bridge
Figure 5. Decayed piles under a 20 TN Bridge